1. Field of the Invention
In the art of pumping liquids it is frequently desirable to provide precise flow rates. A comparatively recent application of pumps is for the controlled flow of medication into humans or animals for the treatment of a variety of physiologic dysfunctions or disease. These medication pumps are being used both externally to the living body, as well as implanted within the living body.
2. Description of the Contemporary and/or Prior Art
With the acceptance of both external and implantable infusion devices, researchers have been attempting to develop a pump which satisfies the strict performance requirements imposed on these devices. It can be seen from an article written by W. J. Spencer entitled "A Review of Programmed Infusion Delivery Systems" (IEEE Transactions on Biomedical Engineering, Vol. BME-28, No. 3, March 1981) that the ideal pump would provide uniform delivery of medication in a reliable and reproducible manner with a minimum of power, weight, and volume. In an effort to meet these requirements the above-referenced article points out that current researchers are experimenting with syringe, peristaltic, piezoelectric and bellows type pumps.
The bellows or positive displacement pump appears to be an attractive type of pump for infusion devices. A patent issued to Peer M. Portner et al (U.S. Pat. No. 4,265,241, issued May 5, 1981) discloses a bellows pump consisting of a piston bellows which is actuated by a solenoid controlled armature. Movement of the piston bellows tends to increase or decrease the volume of the pumping chamber. When the volume of the pump chamber is a maximum, medication is forced from a reservoir, which is maintained at positive pressure, through an input check valve into the pump chamber. When the bellows piston is actuated, the pump chamber is at a minimum volume and fluid is forced out of the chamber through an output check valve. A patent issued to R. E. Fishell (U.S. Pat. No. 4,373,527, issued Feb. 15, 1983) describes a bellows pump which uses the pumping action of the bellows to draw medication from a reservoir through an input check valve and into the pump chamber. When the bellows is extended by a solenoid, medication in the pumping chamber is forced from the pumping chamber via an output check valve. Similarly, U.S. Pat. No. 4,360,019, issued on Nov. 23, 1981 to Portner et al, describes a positive displacement pump which uses an elastometric diaphragm which is driven by a solenoid via a plunger. Movement of a diaphragm varies the volume in the pump chamber which causes fluid to flow into the chamber via a spring loaded input valve or to flow from the chamber via a spring loaded output valve. U.S. Pat. No. 4,152,098, issued to Norman F. Moody et al, on May 1, 1979, discloses a pump having a diaphragm which forms the inlet valve, outlet valve, and is the moveable member which varies the volume in the pumping chamber. A solenoid actuated ball is driven in contact with the diaphragm to vary the volume in the pumping chamber. Although the diaphragm remains in conformity with the ball, differential pressure across the input valve will cause the stroke volume of this prior art pump to vary.
Several of the above-cited references teach the use of a compliant diaphragm or bellows which result in variations in pump stroke volume with changes in the reservoir pressure or in ambient pressure conditions. Variations in reservoir or ambient pressure conditions will cause the diaphragm or the bellows to be deformed such that variations between the actuation and rest volumes of the pump chamber can vary considerably. Similarly, all of the above-cited references teach the use of input and output valves. Since the flow rate through the valve depends upon differential pressure across the valve, the flow rate through the input valve and therefore the stroke volume is dependent upon ambient pressure and reservoir pressure. Therefore, the prior art research and experimentation with various types of pumps has not provided a positive displacement pump which has a constant stroke volume and which is independent of ambient and reservoir pressures.
Prior research also indicates that bellows, or positive displacement pumps, are inoperable if bubbles enter the pumping chamber. If a large enough bubble enters the pumping chamber it will stop the pumping of an incompressible liquid because the gas can compress fully before sufficient pressure is generated to open the inlet and/or outlet valves. Prior art references such as U.S. Pat. No. 4,191,181, issued to Manfred Franetzki et al, on Mar. 4, 1981, attempts to solve this problem by using a wicklike member composed of lightly packed glass-like fibers which have sufficient capillary forces to prevent gas from entering the fine channels. Similarly, U.S. Pat. No. 4,360,019, referenced earlier, attempts to solve this problem by using a looped tube that terminates a short distance from the side of the straight portion of the tube. This distance, is smaller than the diameter of air bubbles, thus blocking their entry into the tube. However, the difficulty with these prior art techniques is that particulates formed in the reservoir chamber can block the filter or wick after a period of operation and thereby block medication delivery.